1. Field of the Invention
This invention relates to the production of complex components made of metallic or ceramic materials, with powders being used as the starting materials. This invention more particularly relates to an improved process for encapsulating a shaped body in preparation for hot isostatic pressing by coating the body with glass using a sol-gel technique.
2. Description of the Related Art
Powders are used as a starting material in numerous production methods in the metallurgical and ceramic industry. Many manufactured shapes, bodies and parts begin with a fine metallic or ceramic powder. At the start of the manufacturing process, these powders are normally consolidated into a shaped body, generally referred to as a "green body" or "preform." Preforms are made by a variety of known techniques, such as die pressing, cold isostatic pressing, slip casting, extrusion, or injection molding. In the course of the manufacturing process, the starting powders are typically combined with additives, such as binders, lubricants, and sintering aids. Preforms are produced in a shape and size that is close to the desired final shape and size of the body or part being manufactured. The preforms, however, are often mechanically weak and porous.
One common method for consolidating the preform to the required density and shape is known as hot isostatic pressing ("HIP"). During the HIP process, a preform is placed in a pressure vessel, wherein it is heated at a high temperature under high pressure, using a gas such as argon or nitrogen. HIP units frequently operate at temperatures in excess of 2000.degree. C., and at pressures of over 200 MPa.
Because the HIP process is conducted at high temperature and pressure at the same time, the process allows for the production of shapes, bodies and parts that would be difficult, if not impossible, to produce using more conventional means, such as sintering. Since the preform is porous, however, and since the HIP process requires the use of high pressure gas, steps must be taken to prevent the gas from entering the porous preform body. The preforms, therefore, must be sealed with a gas-tight capsule prior to pressurization during the HIP process. In addition, the capsule must be capable of withstanding the high temperatures and pressures associated with the HIP process.
Others have attempted to solve the problem of preform encapsulation by utilizing capsules made of either metal or glass. One such method for encapsulating a preform is known as metal canning. This method requires the fabrication of a metal can in a shape close to the desired shape of the body being manufactured. The metal can is filled with powder and sealed under vacuum. The metal canning technique is problematic, however, when the body being produced is complex in shape or size, because of the difficultly and added expense associated with fabricating a can having similar physical features.
Another known method for encapsulating a preform is by placing the preform in a glass capsule, and then sealing the capsule under vacuum. Like the metal canning method, however, the glass encapsulation method is also difficult to use when the porous preform body has an intricate or uncommon shape.
Still another method for encapsulating a porous preform is to place the preform in a bed of glass powder, and heat the powder until the glass is liquified. After the preform is completely immersed in the liquid glass, the pressure from the HIP may be applied. The increase in cost and complexity associated with this method, however, make the glass bath method undesirable.
It is also known to encapsulate a preform by coating the preform with a glass slurry, and, following the coating, heating the preform under vacuum to drive off the binder and produce a gas tight coating. The glass slurry method, however, has proven to be unreliable in that while the coating is drying, cracks in the coating are likely to occur. Moreover, defects are liable to occur during the removal of the binder by thermal decomposition. Finally, the glass slurry method also adds to the complexity and cost of the manufacturing process.
A sol-gel method may be utilized to produce a thin film of glass. A sol-gel method generally comprises hydrolyzing and polycondensating a metal alkoxide in a solution containing the metal alkoxide, water and an alcohol to form a porous solid material, and heating the porous solid material to produce a glass material. U.S. Pat. No. 5,368,887 (Hoshino), for example, describes one type of process for producing a thin glass film by a sol-gel method. The '887 patent, however, neither shows nor suggests the use of the sol-gel derived glass for encapsulating a preform for the HIP process.